Multicrystalline silicon brick and silicon wafer therefrom

ABSTRACT

Present disclosure provides a multicrystalline silicon (mc-Si) brick, including a bottom portion starting from a bottom to a height of 100 mm, a middle portion starting from the height of 100 mm to a height of 200 mm; and a top portion starting from the height of 200 mm to a top. A percentage of incoherent grain boundary in the bottom portion is greater than a percentage of incoherent grain boundary in the top portion. Present disclosure also provides a multicrystalline silicon (mc-Si) wafer. The mc-Si wafer includes a percentage of non-Σ grain boundary from about 60 to about 75 and a percentage of Σ3 grain boundary from about 12 to about 25.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of prior-filed application Ser. No.14/698,615, filed Apr. 28, 2015, under 35 U.S.C. 120, and incorporatesthe prior-filed application by reference in its entirety.

BACKGROUND

Multi-crystalline silicon (mc-Si) grown by directional solidificationhas attracted much attention in photovoltaic industry because of its lowproduction cost and high throughput. However, the crystal qualitydeteriorates as the ingot grows taller due to the accumulation ofimpurities and the generation (multiplication) of dislocations. Becausethese defects, as well as crystal properties, are affected by grainmorphologies and lattice orientations, the control of grain structuresis important during crystal growth.

Different from random grain boundaries, special grain boundaries arecharacterized by particular misorientation and extensive areas of goodfit (special grain boundaries are described by a sigma number (1<Σ<29),which is defined as the reciprocal of the fraction of lattice points inthe boundaries that coincide between the two adjoining grains on thebasis of the coincident site lattice (CSL) model.). Thus, there is lowdistortion of atomic bonds and relatively little free volume for specialgrain boundaries and consequently low boundary energy.

BRIEF DESCRIPTION OF THE DRAWINGS

The file of this patent or application contains at least one drawingexecuted in color. Copies of this patent or patent applicationpublication with color drawings will be provided by the Patent andTrademark Office upon request and payment of the necessary fee.

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1A is a schematic illustration of the experimental setup, inaccordance with some embodiments.

FIG. 1B is a schematic illustration of silicon beads in thenitride-coated quartz crucible, in accordance with some embodiments.

FIG. 2A is a longitudinal cross section of ingots at a pulling speed of10 mm/h, and the dashed line indicates the initial melt/solid interface,in accordance with some embodiments.

FIG. 2B is a longitudinal cross section of ingots at a pulling speed of50 mm/h, and the dashed line indicates the initial melt/solid interface,in accordance with some embodiments.

FIG. 2C is a longitudinal cross section of ingots at a pulling speed of200 mm/h, and the dashed line indicates the initial melt/solidinterface, in accordance with some embodiments.

FIG. 3A is a grain structure (left), EBSD mapping (bottom), and theinverse pole diagrams (right) at an ingot height 0 mm of ingot V, inaccordance with some embodiments.

FIG. 3B is a grain structure (left), EBSD mapping (bottom), and theinverse pole diagrams (right) at an ingot height 7 mm of ingot V, inaccordance with some embodiments.

FIG. 3C is a grain structure (left), EBSD mapping (bottom), and theinverse pole diagrams (right) at an ingot height 14 mm of ingot V, inaccordance with some embodiments.

FIG. 4 is a comparison of major grain orientations at different growthdistances for different pulling speeds; the inverse pole diagram of theorientation of V20 at h=19 mm is included for comparison, in accordancewith some embodiments.

FIG. 5 shows the development of major grain boundaries along the growthdirection for ingot V1. The twin boundaries (purple) and typical grainboundary mappings at the given heights are inserted for comparison; theresults for ingots V5 and V20 are similar. (For interpretation of thereferences to color in this figure caption, the reader is referred tothe web version of this paper), in accordance with some embodiments.

FIG. 6A shows a grain competition mechanisms (ingot V1, overgrown), ineach figure, from the left to the right are EBSD, grain boundarymappings, and the inverse pole diagrams from different ingot positions.The number on the EBSD mapping indicates the wafer number, which wasclosed to its height in mm, in accordance with some embodiments.

FIG. 6B shows grain competition mechanisms (ingot V1, alow-interfacial-energy grain formation at the tri-junction), in eachfigure, from the left to the right are EBSD, grain boundary mappings,and the inverse pole diagrams from different ingot positions. The numberon the EBSD mapping indicates the wafer number, which was closed to itsheight in mm, in accordance with some embodiments.

FIG. 7A shows grain competition mechanisms (ingot V1,high-interfacial-energy grain from the tri-junction with twin boundarymovement), in each figure, from the left to the right are EBSD, grainboundary mappings, and the inverse pole diagrams from different ingotpositions. The number on the EBSD mapping indicates the wafer number,which was closed to its height in mm, in accordance with someembodiments.

FIG. 7B shows grain competition mechanisms (ingot V1, twins movement),in each figure, from the left to the right are EBSD, grain boundarymappings, and the inverse pole diagrams from different ingot positions.The number on the EBSD mapping indicates the wafer number, which wasclosed to its height in mm, in accordance with some embodiments.

FIG. 8 shows the grain size at different heights of Ingot A and Ingot B,in accordance with some embodiments.

FIG. 9A shows grain orientation mappings and typical grain boundarymappings at a given height (95 mm) of Ingot B, in accordance with someembodiments.

FIG. 9B shows defect location mapping and typical grain boundarymappings at a given height (95 mm) of Ingot B, in accordance with someembodiments.

FIG. 10A shows grain orientation mappings and typical grain boundarymappings at a given height (132.5 mm) of Ingot B, in accordance withsome embodiments.

FIG. 10B shows defect location mapping and typical grain boundarymappings at a given height (132.5 mm) of Ingot B, in accordance withsome embodiments.

FIG. 11A shows grain orientation mappings and typical grain boundarymappings at a given height (170 mm) of Ingot B, in accordance with someembodiments.

FIG. 11B shows defect location mapping and typical grain boundarymappings at a given height (170 mm) of Ingot B, in accordance with someembodiments.

FIG. 12A shows grain orientation mappings and typical grain boundarymappings at a given height (207.5 mm) of Ingot B, in accordance withsome embodiments.

FIG. 12B shows defect location mapping and typical grain boundarymappings at a given height (207.5 mm) of Ingot B, in accordance withsome embodiments.

FIG. 13 shows the percentage of various grain orientations at givenheights of Ingot B, in accordance with some embodiments.

FIG. 14 shows the percentage of various grain boundary types at givenheights of Ingot B, in accordance with some embodiments.

FIG. 15A shows grain orientation mappings and typical grain boundarymappings at a given height (95 mm) of Ingot A, in accordance with someembodiments.

FIG. 15B shows defect location mapping and typical grain boundarymappings at a given height (95 mm) of Ingot A, in accordance with someembodiments.

FIG. 16A shows grain orientation mappings and typical grain boundarymappings at a given height (132.5 mm) of Ingot A, in accordance withsome embodiments.

FIG. 16B shows defect location mapping and typical grain boundarymappings at a given height (132.5 mm) of Ingot A, in accordance withsome embodiments.

FIG. 17A shows grain orientation mappings and typical grain boundarymappings at a given height (170 mm) of Ingot A, in accordance with someembodiments.

FIG. 17B shows defect location mapping and typical grain boundarymappings at a given height (170 mm) of Ingot A, in accordance with someembodiments.

FIG. 18A shows grain orientation mappings and typical grain boundarymappings at a given height (207.5 mm) of Ingot A, in accordance withsome embodiments.

FIG. 18B shows defect location mapping and typical grain boundarymappings at a given height (207.5 mm) of Ingot A, in accordance withsome embodiments.

FIG. 19 shows the percentage of various grain orientations at givenheights of Ingot A, in accordance with some embodiments.

FIG. 20 shows the percentage of various grain boundary types at givenheights of Ingot A, in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

In some embodiments of the present disclosure, special grain boundarieswith sigma numbers smaller than or equal to 3 are referred to ascoherent grain boundaries. On the other hand, special grain boundarieswith sigma numbers greater than 3 are referred to as incoherent grainboundaries, and special grain boundaries with sigma numbers greater than27 are referred to as non-sigma grain boundaries. In some embodiments,the incoherent grain boundaries and the non-sigma grain boundaries arecollectively called “non-coherent grain boundary”.

In some embodiments of the present disclosure, a silicon brick is aportion of a silicon ingot. For example, a silicon brick can be a 156 mmby 156 mm column separated from a silicon ingot. In some cases, asilicon ingot can be divided into a 5 by 5 silicon brick array. For anindustrial practice, each of the silicon brick can be further slicedinto about 600 silicon wafers. Polysilicon has a melting point of about1,414 degrees Celsius, and the aforesaid separation operation of thesilicon brick from the silicon ingot or the silicon wafer from thesilicon brick can only generate frictional heat lower than about 100degrees Celsius. Hence, said separation can be considered as purephysical change (decrease in dimension) without the involvement of anychemical change, for a chemical change can only occurs at a temperaturein proximity to the melting point of the silicon.

In some embodiments of the present disclosure, three portions of asilicon ingot, or a silicon brick separated from a silicon ingot, can beidentified according to a height of the ingot or brick. Generally, abrick of an ingot can be equally divided into three portions. In someembodiments, a bottom portion ranges from a bottom to a height of 100mm; a middle portion ranges from the height of 100 mm to a height of 200mm; and a top portion ranges from the height of 200 mm to a top. Variouscrystal properties such as coherency of grain boundary, grainorientations, or grain size can be separately characterized according todifferent aforesaid portions.

The simplest way to control crystal structure is to use seeds with givenorientations, and the use of mono-crystalline seeds has become popularin recent years for the production of the so-called mono-like orquasi-mono ingots. Unfortunately, the grain competition and new grainformation may spoil the structures and reduce the production yield.Therefore, it is believed that using a preferred growth orientation fordirectional solidification could increase the structure yield and reducedefect density. This issue can be discussed based on the twin formationmechanisms, and concluded that {100} orientation turned out to be themost difficult one to the growth of mono-like ingots.

To obtain better crystal properties in mc-Si growth, one of theapproaches is the so-called dendrite casting method, which controls theinitial undercooling to induce [110]/[112] dendrites and Σ3 grainboundaries. However, the control of undercooling is not so easy in acommercial growth station due to the large thermal resistance from thethick bottom of the quartz crucible and the imperfect nitride coating.This thus limits the applications of the method for mass production.

For growing mc-Si, it is found that the grains seemed to be orientedrandomly, but the relative grain orientation could be described mostlyby special coincidence orientations. This indicated that the grainstructure developed from far fewer independent nuclei that were decidedat the initial stage of nucleation and crystal growth. It is believedthat {111} was the preferred growth orientation for silicon due to thesmaller interfacial energy. The melt growth behavior of me-Si using anin situ monitoring system during a thin-film directional solidificationwas studied, and it is observed that different growth behaviors oforiented grains appear in different cooling conditions.

Conventionally, Σ3 grain boundaries are more desired than a non-coherentor non-Σ grain boundaries in a mc-Si structure due to the fact that Σ3grain boundaries are more coherent and functions as a less efficientrecombination center compared with the non-coherent counterpart(including incoherent and non-Σ grain boundaries). Alternatively stated,Σ3 grain boundaries are more “electrically inert” than the non-coherentboundaries. Using conventional means to grow mc-Si, the percentage of Σ3grain boundaries is greater than the percentage of the non-coherentcounterpart in order to retain sufficient quantum efficiency orconversion efficiency of a photovoltaic. However, the quantum efficiencyof a mc-Si photovoltaic still hits a limit because the accumulation ofthe impurities and multiplication of dislocations not only occurs at thegrain boundary region but also in the grain body. Taking this fact intoconsideration, the present disclosure provides a mc-Si structure havinga greater percentage of non-coherent grain boundaries than that of thecoherent grain boundaries, and showing better conversion efficiency thanthe me-Si structure prepared according to conventional means.

In some embodiments of the present disclosure, the coherency of thegrain boundary can be identified by at least two methods: (1) by acomputer-programmed EBSD, and (2) by photoluminescence (PL) observingthe incoherent/non-Σ grain boundaries.

In some embodiments, a structure of mc-Si, which can be a mc-Si ingot, ame-Si brick, or a me-Si wafer, is disclosed. The structure of me-Sishows a greater percentage of non-coherent grain boundaries (forexample, a summation of the incoherent grain boundaries and non-Σ grainboundary) than that of the coherent grain boundaries. In someembodiments, the me-Si brick possesses a greater percentage ofnon-coherent grain boundary in a bottom portion than that in a topportion. In some embodiments, the me-Si wafer possesses a percentage ofnon-Σ grain boundary from about 60 to about 75 and a percentage of Σ3grain boundary from about 12 to about 25. In some embodiments, apercentage of the non-Σ grain boundary and a percentage of Σ3 grainboundary of a me-Si wafer are substantially identical.

A method for obtaining the me-Si structure described herein is alsodisclosed. A nucleation promotion layer is utilized to promote a smallgrain size at the initial of the me-Si grain growth. As described in thefollowing, the nucleation promotion layer can be made of silicon beadswith an average dimension of about less than 10 mm. In some embodiments,the silicon beads can take spherical shape. In some embodiments, thesilicon beads can be single crystalline silicon, multicrystallinesilicon, silicon carbide, or combinations thereof.

In some embodiments of the present disclosure, when preparing a me-Siingot or brick, {100} and {110} poly-silicon grains were favored at ahigh cooling rate, e.g., 30 K/min, as a result of kinetic control; thegrowth velocity of {100} was 140.8 cm/h at 30 K./min. On the contrary,at a low cooling rate, e.g., 1 K/min, {111} grains were dominant due tothermodynamic control that favors the orientation with the lowestinterfacial energy. By using phase field modeling, similar developmentscan be obtained. The force balance is further used at the tri-junctionto explain the dominance of {111} grains at the low growth rate. Thecritical velocity for facet formation, as a result of morphologicalinstability, was estimated around 12 cm/h. However, the growth velocityfor {100} dominated growth was unknown. Therefore, for the graincompetition in a normal speed at about 1 cm/h in commercial mc-Siproduction, {111} grains should be dominant. Moreover, the graincompetition in silicon is far more complicated than people havingordinary skill in the art have expected due to twin formation.

Referring to FIG. 1A, in some embodiments, mc-Si ingot (70 mm inbroadest dimension) is grown by directional solidification in anapparatus 100. The directional solidification setup using induction coil101 for heating is shown, where the crucible 102 is insulated bygraphite felt 103 [to inventor: please verify whether 103 is an aluminumfelt or a graphite felt] to better control the solidification front.Spherical silicon beads 104 (for example, 0.92 mm in diameter from CV21, Japan) are used as the seed layer. In some embodiments, thenucleation promotion layer may possess a height H at a bottom of thecrucible. In some embodiments, the height H is about 20 mm.

In some embodiments of the present disclosure, spherical silicon beadscan be used as the nucleation promotion layer for directionalsolidification of me-Si. However, silicon beads are not limited to aspherical shape. Any form of silicon scraps with a characteristicdimension of equal to or smaller than 10 mm is within the contemplatedscope of the present disclosure. For example, a roughened cruciblebottom can be used as the nucleation promotion layer. In someembodiments, the roughened crucible bottom can be formed by a blanketphysical or chemical etch and thus the concave and convex patterns beingrandomly disposed, with a characteristic dimension (for example, adistance between a vertex of a convex and a bottom of a concave) beingsmaller than or equal to about 10 mm. In other embodiments, theroughened crucible bottom can be formed by a patterned etch. Forexample, line features or dot features with a pitch of smaller than orequal to about 10 mm can be formed as the nucleation promotion layer.

In the following description, the experimental setup and procedure aredescribed briefly, followed by results, discussion, and conclusion. Insome embodiments, the seeds used for directional solidification mc-Sigrowth are not limited to spherical beads, as discussed. Any beadshaving an average diameter of lower than 50 mm, preferably lower than 10mm, are suitable for the subsequent mc-Si growth. In some embodiments,the silicon beads can be made of single crystalline silicon,multicrystalline silicon, or a mixture thereof. Other materials such assilicon carbide can also be used, separately or together, with siliconseeds. In the case of using single crystalline beads, although all thebeads are having a single orientation, for example, {110}, the poledirection of each bead is not necessary perpendicular to a normal of thebottom of the mc-Si ingot or brick. Details of the single crystallinebeads will be further discussed in FIG. 1B.

Referring to FIG. 1B, in some embodiments with single crystallinesilicon beads, for example, {110} silicon beads, the pole direction P1of a first bead 110 and a normal N of the crucible bottom 102B form anangle θ1 greater than zero. Similarly, the pole direction P2 of arandomly selected second bead 120 (other than the first bead 110) and anormal N of the crucible bottom 102B form an angle θ2 greater than zero.The angles θ1 and θ2 can be different. In other words, even if the beads110 and 120 are made of single crystalline materials having anorientation of {110}, the ingot formed thereon can be multicrystallinewith random crystal orientations due to the randomly disposed poledirections of the single crystalline beads. In other embodiments, thesilicon beads can be multicrystalline silicon, silicon carbide,crystalline material beads other than silicon, or combinations thereof.

In some embodiments, before solidification started, silicon raw materialwas melted leaving about 5 mm to 10 mm of the nucleation promotion layerat the bottom. The temperature gradient of the furnace for crystalgrowth was about 10 K/cm. Therefore, the estimated cooling rate wasabout 3.33 K/min for the crucible speed of 20 cm/h. However, in acommercial mc-Si production, the temperature gradient of the furnace isabout 1 K/cm and the crucible speed being around 1 cm/h. Hence theestimated cooling rate is about 0.0167 K/min in a production setting.

Referring to FIG. 2A to FIG. 2C, three ingots are grown from differentcrucible pulling speeds. The ingots are labeled as V1 for the cruciblespeed at 10 mm/h, V5 for 50 mm/h, and V20 for 200 mm/h. After crystalgrowth, the ingots were cut into wafers and polished for furtheranalysis. The wafers were also chemically etched (HNO3:HF=6:1) forsubsequent characterizations. The grain orientation and boundarymappings were carried out by using electron backscattered diffraction(EBSD) (Horiba Nordlys F+) with a step size of 10 μm, which wasinstalled in an SEM (Hitachi S3400).

The longitudinal cross sections of the grown ingots are shown in FIG. 2Ato FIG. 2C for comparison. As shown, the columnar grains were grownupward nicely from the nucleation promotion layer in all cases. Theinterface shape was nearly flat except near the crucible wall. Thiscould also be seen from the grain growth direction. Near the cruciblewall (not shown), new grains appeared and grew inwards. As the pullingspeed increased, the interface became more concave. Again, this could beseen from the convergent grains toward the center near the upper partsof FIG. 2B and FIG. 2C. After crystal growth, the unmelted silicon beadssintered together, but the initial solidification front could bedetermined from the starting points of the columnar grains.

The width of the columnar grains grew upward slowly, but the grain sizeneeded to be analyzed from the cross section grains, which will bediscussed shortly. In some embodiments, the columnar grains can have anaverage height of about 3 cm. However, in a commercial productionsetting, the columnar grains can have an average height of from about 25cm to about 36 cm due to different ingot growth conditions. Aninteresting observation was that some grains were terminated suddenly byother grains due to their tilted growth orientation from the observedcutting plane. Moreover, some disoriented grains grew in the directionthat was quite different from the growth direction. This could beexplained by the twin formation from the {111} facets, which will bediscussed shortly. Otherwise, the oriented grains will grow over thedisoriented grains during grain competition

The horizontal cuts of ingot V1 and their EBSD results are shown in FIG.3A-FIG. 3C, respectively, for the positions at ingot or brick height h=0mm, 7 mm, and 14 mm from the starting point of the columnar grains.Although the area near the wafer center (50% of the diameter), where thegrains grew vertically, should be the better area for orientationanalysis, its difference from the bigger area mapping (80% of thediameter) was found not significant change. To have more grains foranalysis, the present disclosure still chose the bigger area fororientation comparison. In each figure, the grains in the dashed box areanalyzed. Their orientation mapping is shown at the bottom of thefigure; the corresponding color to the orientation is indicated at thetop right corner of the photograph in FIG. 3A. In addition, the inversepole diagrams, in terms of the frequency counts and their contours, areshown on the right. As shown in FIG. 3A, the grains are uniform andround due to the uniform silicon beads are used as the nucleationpromotion layer. The orientations were quite random as well, althoughthe contours showed some difference, but the difference of the contourminimum (0.78) shown in blue and maximum (1.22) shown in red is notlarge. Also, the horizontal cut did not follow exactly the initialsolidification front. However, as the position increased to h=7 mm, asshown in FIG. 3B, the grain size increased. More importantly, thepercentage of the orientations near {112} and {111} increasedsubstantially. This trend continued to the top of the ingot, where theorientations near {112} being dominant, as shown in FIG. 3C.

The reason for the dominance of {112} grains may related to the factorsthat this orientation has the lowest interfacial energy next to {111},and the angle between {111} and {112} is only 19.471. Some commercialwafers grown by using an incubation layer also have more {112} grainstill the top of the ingot. In other words, the grain competitionremained similar regardless the ingot height; the percentage of {112} ath=14 mm for V1 was about 15%. Furthermore, in the development of grainstructures in a small notch, {112} grains became dominant form theinitial {110} grains in a small growth distance of 4 mm. Therefore, insome embodiments, {112} are the dominance orientation of the graincompetition from random seeds.

The development of grain structures of ingots V5 and V20 was similar toingot V1. However, {111} became more dominant at the end of the growth.The percentage evolutions of major grain orientations of the threeingots were compared in FIG. 4. As shown, {112} grains became dominantfor ingot V1, while {111} grains became dominant for ingots V5 and V20.In fact, {212} grains in V5 were slightly more than {111} grains, butthe smallest orientation difference between them is only 15.81, and thiscould be caused by the deflection of the interface. Again, the dominanceof {111} grains was consistent with previous observations. The {110}grains in V1 and V20 were also more than other grains. Because the {110}orientation is rather rough, the growth rate in this orientation couldbe high at high undercooling. Nevertheless, the distribution of thegrain orientations in all cases was rather wide, but {100} grainsremained few near the end. As will be discussed shortly, the anglebetween {100} and {111} is large being about 54.71, which is easier togenerate a higher undercooling for twin formation.

The average grain sizes are also calculated. For simplicity, the grainsize is calculated by dividing the diagonal distance of the dashed boxby the grain numbers across the distance. The grain size increased withthe growth distance for all velocities; it increased from 0.92 mm at thebottom to about 1.2, 1.4 and 1.6 mm at the height of 19 mm for ingotsV1, V2, and V3, respectively. In some embodiments, it is found that thegrain growth became more significant with the increasing cruciblepulling velocity.

The development of grain boundaries was further examined, and the resultfor ingot V1 is shown in FIG. 5. The mappings of grain and twinboundaries at different positions are inserted in the figures forcomparison; the twin boundaries are indicated by the purple lines. Asshown, the percentage of non-Σ grain boundaries was quit high beingabout 60-70% at the beginning, but decreased slowly to about 45% nearthe end of the growth. In some embodiments, the percentage of non-Σgrain boundaries is at about 70% at the beginning of the growth or atthe bottom of the ingot, and such percentage drops to about 40% within asubsequent 20 mm growth height.

Still referring to FIG. 5, the percentage of Σ3 grain boundaries wasonly about 20-25% at the beginning, but their percentage increased withheight. Near the top of the ingot, the percentage was about 40-45%. Thepercentage of non-Σ grain boundaries was about 65-75% at the beginning,but their percentage decreases with height. Near the top of the ingot,the percentage was about 40-45%, similar to that of the Σ3 grainboundaries. In some embodiments, a percentage of non-Σ grain boundaryand a percentage of Σ3 grain boundary of a multicrystalline silicon(mc-Si) wafer are substantially identical. Further in some embodiments,the percentage of non-Σ grain boundary and the percentage of Σ3 grainboundary are in a range of from about 40 to about 50. In someembodiments, the aforesaid mc-Si wafer can be separated from a topportion of an ingot or a brick. In some embodiments, the twin boundariesand Σ3 boundaries in a mc-Si wafer are almost the same based on thecomputer software of EBSD. In some embodiments, a preferred grainorientation of a mc-Si wafer is observed to include {112}.

Apparently, the high percentage of the non-Σ or incoherent boundarieswas due to the initial nucleation from the silicon beads, which hadrandom orientations. Some twins existed already in the silicon beads dueto their formation process. As crystal growth continued, grainboundaries with a higher symmetry and a lower interfacial energy, suchas the coherent Σ3 and twin grain boundaries, are preferred. Ingots V5and V20 had very similar grain boundary evolution as ingot V1, as shownin FIG. 5. However, the coherent Σ3 grain boundaries increased faster asthe pulling speed increased, and for ingot V3 (not shown in FIG. 5), thecoherent Σ3 grain boundaries increase to more than 40% within 10 mm ofgrowth height. Again, this could be due to the increase of undercoolingin the groove of grain boundaries for twin nucleation.

However, in other embodiments where the scale of the crucible and thetemperature gradient are inclined to fit an industrialized productionsetting, a percentage of the non-coherent grain boundary in a bottomportion of a me-Si brick or ingot is greater than a percentage of thenon-coherent grain boundary in a top portion of a me-Si brick or ingot.In some embodiments, the non-coherent grain boundary includes non-Σgrain boundaries as previously discussed. Moreover, as shown in FIG. 5,the non-coherent grain boundary herein may include Σ5, Σ9, Σ27, other Σ,and non Σ grain boundaries. On the other hand, a percentage of coherentgrain boundary in the bottom portion of a mc-Si brick or ingot is lowerthan a percentage of the coherent grain boundary in a top portion of ame-Si brick or ingot. In some embodiments, the coherent grain boundaryincludes Σ3 grain boundary. As shown in FIG. 5, the coherent grainboundary herein may include both Σ3 and twin grain boundaries.

Grain competition and the development of twin boundaries from the wafersof ingot V1 is shown in the following description. Four cases areobserved as shown in FIG. 6A, FIG. 6B, FIG. 7A, and FIG. 7B. The firstcase is illustrated in FIG. 6A for the grain bounded by non-coherentboundaries, and an orientation mapping is shown at the right of thefigure. The grain with a higher interfacial energy, i.e., {115}, isovergrown by others having a lower interfacial energy as shown by thesequence 6, 7, 8, 9 of the EBSD mapping and grain boundary (GB) mapping.In the second case, as shown in FIG. 6B, new grains having a lowerinterfacial energy could nucleate from the tri-junction. A {111} grainappeared at the tri-junction, accompanied by the formation of a twinboundary shown by the sequence 14, 15, 16, 17 of the EBSD mapping andgrain boundary (GB) mapping. The third case is illustrated in FIG. 7Afor the nucleation of a high-interfacial-energy grain {100} from atri-junction. The {221} grain was overgrown, shown by the sequence 7, 8,9, 10, 11 of the EBSD mapping and grain boundary (GB) mapping. The lastcase in FIG. 7B is the formation of grains between two twin boundariesthat appeared and disappeared with the movement of twin boundaries shownby the sequence 2, 3, 4, 5, 6, 7 of the EBSD mapping and grain boundary(GB) mapping. The formation of multiple twins was consistent with the insitu observation using X-ray topography.

Apparently, the first two cases in FIG. 6A and FIG. 6B led to thedominance of low-interfacial-energy grains, such as {111}, during graincompetition, as well as the formation of twin boundaries. However, inthe last two cases in FIG. 7A and FIG. 7B, in addition to the formationof twin boundaries, the high-interfacial-energy grains, such as {100},could also be generated. According to calculation, {100} could begenerated from the twining of {221}. Moreover, although the twiningprocess could generate high-interfacial-energy grains, the twiningprocess could continue. Because {100} grains have more tilted {111}facets, 54.71, the undercooling could be higher and the twin formationcould be easier. As a result, near the end of the growth, the percentageof {100} grains was very low. For ingots V5 and V20, the mechanisms ofthe development of grain structures were found similar to that of ingotV1, as shown in FIG. 6A to FIG. 7B. Again, the increase of twin andnon-coherent grain boundaries is consistent with the twin formationmechanism at the facets. The wide distribution of the grain orientationmight also be explained by the same mechanism as well.

The preferred growth orientation of me-Si in directional solidificationby using small silicon beads as the nucleation promotion layer withrandom orientations can be observed. It is found that {112}/{111} becamedominant quickly in a short distance at the low crucible pulling speedof 1 cm/h. As the pulling speed increased, grains with an orientationnear {111} became dominant, but the distribution was still wide. On theother hand, the percentage of {100} grains is low in all cases. Due tothe random nucleation promotion layer orientations, the initialpercentage of non-coherent grain boundaries was high being about 70%. Asthe crystal growth proceeded, more twin boundaries appeared, and theirgrowth rate increased slightly with the increasing pulling speed. Theseobservations were explained by the minimization of interfacial energy,as well as the twin nucleation/growth from {111} facets.

Referring to FIG. 8 and Table 1, two ingots (ingot A and ingot B) areprepared according to the method disclosed herein. Table 1 records thegrain sizes measured at different heights and FIG. 8 demonstrates themeasurement result from a brick separated from ingot A and ingot B,respectively. A grain size distribution at different brick heights isshown. In both ingot A and ingot B, the grain size monotonicallyincreases from a bottom portion of the brick to a top portion of thebrick. For ingot A, the grain size (15.76 mm) at a brick height of 245mm is 50% more than the grain size (10.26 mm) at a brick height of 95mm. For ingot B, the grain size (16.97 mm) at a brick height of 207.5 mmis 45% more than the grain size (11.61 mm) at a brick height of 95 mm.

Referring to FIGS. 9A, 10A, 11A, 12A, and Table 1, a grain orientationmapping and a grain boundary mapping at a given height (95 mm in FIG.9A, 132.5 mm in FIG. 10A, 170 mm in FIG. 11A, 207.5 mm in FIG. 12A) ofIngot B are shown. Table 1 records the ratio (in percentage) of grainorientations at different heights of a brick separated from ingot B.Grain orientations and grain boundary types are color-coded according tothe annotations on the right of the figures. Referring to FIGS. 9B, 10B,11B, 12B, and Table 1, a defect location mapping and a grain boundarymapping at a given height (95 mm in FIG. 9B, 132.5 mm in FIG. 10B, 170mm in FIG. 11B, 207.5 mm in FIG. 12B) of Ingot B are shown. Table 1records the ratio (in percentage) of grain boundary types at differentheights of a brick separated from ingot B. Defects (shown in black) andgrain boundary types are color-coded according to the annotations on theright of the figures.

Referring to FIG. 13, FIG. 13 shows a quantitative summary diagram inaccordance with FIGS. 9A, 10A, 11A, and 12A. Distribution of the grainorientations at different heights of Ingot B is shown in FIG. 13. Insome embodiments, the preferred orientation at various heights of IngotB is {112}.

Referring to FIG. 14, FIG. 14 shows a quantitative summary diagram inaccordance with FIGS. 9B, 10B, 11B, and 12B. Distribution of grainboundary types at different height of Ingot B is shown in FIG. 14. Theratio (in percentage) of high-angle grain boundary (for example, non-Σgrain boundary) decreases as moving to a higher portion of the ingot(i.e. greater brick height). In some embodiments, the percentage ofnon-Σ grain boundary is from about 65 to about 75 at the bottom portionof an ingot. The percentage of more coherent grain boundary (forexample, Σ3 grain boundary) increases as moving to a higher portion ofthe ingot. In some embodiments, the percentage of Σ3 grain boundary isfrom about 12 to about 18 at the bottom portion of an ingot. There aregrain boundaries for which the coherency is in between Σ3 grain boundaryand non-Σ grain boundary, for example, Σ5, Σ9, Σ27, other Σ(collectively other grain boundary more incoherent than the Σ3 grainboundary). As shown in FIG. 14, the percentage of Σ3 grain boundarybeing lower than the percentage of the non-Σ grain boundary and greaterthan other grain boundary being more incoherent than the Σ3 grainboundary. In some embodiments, the percentage of non-Σ grain boundary isgreater than a summation of the percentage of Σ3 grain boundary and thepercentage of other grain boundary being more incoherent than the Σ3grain boundary.

Referring to FIGS. 15A, 16A, 17A, 18A, and Table 1, a grain orientationmapping and a grain boundary mapping at a given height (95 mm in FIG.15A, 132.5 mm in FIG. 16A, 170 mm in FIG. 17A, 207.5 mm in FIG. 18A) ofIngot B are shown. Table 1 records the ratio (in percentage) of grainorientations at different heights of a brick separated from ingot B.Grain orientations and grain boundary types are color-coded according tothe annotations on the right of the figures. Referring to FIGS. 15B,16B, 17B, 18B, and Table 1, a defect location mapping and a grainboundary mapping at a given height (95 mm in FIG. 15B, 132.5 mm in FIG.16B, 170 mm in FIG. 17B, 207.5 mm in FIG. 18B) of Ingot B are shown.Table 1 records the ratio (in percentage) of grain boundary types atdifferent heights of a brick separated from ingot B. Defects (shown inblack) and grain boundary types are color-coded according to theannotations on the right of the figures.

Referring to FIG. 19, FIG. 19 shows a quantitative summary diagram inaccordance with FIGS. 15A, 16A, 17A, and 18A. Distribution of the grainorientations at different heights of Ingot B is shown in FIG. 19. Insome embodiments, the preferred orientation at various heights of IngotB is {112}.

Referring to FIG. 20, FIG. 20 shows a quantitative summary diagram inaccordance with FIGS. 15B, 16B, 17B, and 18B, Distribution of grainboundary types at different heights of Ingot B is shown in FIG. 14. Theratio (in percentage) of high-angle grain boundary (for example, non-Σgrain boundary) decreases as moving to a higher portion of the ingot(i.e. greater brick height). The position of a multicrystalline silicon(mc-Si) wafer on an ingot can be determined according to resistanceanalysis. In general, a mc-Si wafer in proximity to a bottom of an ingotor a brick has a greater resistance than a mc-Si wafer in proximity to atop of the ingot or the brick. In some embodiments, the percentage ofnon-Σ grain boundary is from about 65 to about 75 in a mc-Si wafer. Inother embodiments, the percentage of Σ3 grain boundary is from about 12to about 25 in a mc-Si wafer. Furthermore, the aforesaid wafer may beseparated from a bottom portion of an ingot or brick. Referring back toFIG. 5, in addition to Σ3 grain boundary, another type of coherent grainboundary, namely twin boundary, can be observed in a mc-Si waferprepared according to the method described herein. In some embodiments,the percentage of the Σ3 grain boundary is substantially the same as apercentage of the twin boundary in a me-Si wafer. The aforesaid wafermay be separated from a bottom portion of an ingot or brick.

Table 1 below provides complementary information for FIG. 8, FIG. 13,FIG. 14, FIG. 19, and FIG. 20 of the present disclosure. The informationencompassed in figures listed above is presented in the context of aTable for clarity.

TABLE 1 Complementary Data of FIG. 8, FIG. 13, FIG. 14, FIG. 19, andFIG. 20 Ingot A Grain boundary type ratio Grain orientation ratioPosition (mm) Grain size (mm) Σ3 Σ5 Σ9 Σ27 Other Σ Non Σ (001) (115)(113) (112) (111) (313) (101) (315) 95 10.26 15% 1% 4% 2% 5% 73% 1% 14%10%  23% 12% 10% 6% 24% 132.5 12.25 18% 0% 4% 3% 7% 68% 1% 14% 7% 23%14% 13% 6% 22% 170 12.98 19% 1% 6% 2% 4% 68% 2% 14% 9% 25% 10% 13% 4%23% 207.5 15.21 17% 0% 6% 2% 6% 69% 1% 19% 9% 23% 12% 14% 4% 18% 24515.76 20% 1% 8% 4% 5% 62% 3% 17% 8% 23% 15% 12% 4% 18% Ingot B Grainboundary type ratio Grain orientation ratio Position (mm) Grain size(mm) Σ3 Σ5 Σ9 Σ27 Other Σ Non Σ (001) (115) (113) (112) (111) (313)(101) (315) 95 11.61 14% 0% 5% 3% 6% 72% 0% 16% 8% 30% 11% 8% 3% 24%132.5 12.25 15% 0% 4% 4% 6% 71% 0% 15% 9% 30% 13% 6% 5% 22% 170 13.3716% 1% 6% 4% 5% 68% 1% 18% 9% 28% 16% 6% 3% 19% 207.5 16.97 18% 1% 5% 3%5% 68% 1% 20% 11%  25% 17% 6% 3% 17%

Some embodiments of the present disclosure provides a multicrystallinesilicon (mc-Si) brick, including a bottom portion starting from a bottomto a height of 100 mm, a middle portion starting from the height of 100mm to a height of 200 mm; and a top portion starting from the height of200 mm to a top. A percentage of incoherent grain boundary in the bottomportion is greater than a percentage of incoherent grain boundary in thetop portion.

In some embodiments, the me-Si brick further including a preferred grainorientation of {112} in the bottom portion, the middle portion, and thetop portion.

In some embodiments, the incoherent grain boundary includes non-Σ grainboundaries.

In some embodiments, a percentage of coherent grain boundary in thebottom portion is lower than a percentage of coherent grain boundary inthe top portion.

In some embodiments, the coherent grain boundary includes Σ3 grainboundary.

In some embodiments, the percentage of Σ3 grain boundary being lowerthan the percentage of the non-Σ grain boundary and greater than othergrain boundary being more incoherent than the Σ3 grain boundary.

In some embodiments, the percentage of non-Σ grain boundaries is fromabout 65 to about 75 at the bottom portion.

In some embodiments, the percentage of non-Σ grain boundary is greaterthan a summation of the percentage of Σ3 grain boundary and thepercentage of other grain boundary being more incoherent than the Σ3grain boundary.

In some embodiments, the percentage of Σ3 grain boundaries is from about12 to about 18 at the bottom portion.

In some embodiments, the mc-Si brick further including a nucleationpromotion layer under the bottom portion, wherein the nucleationpromotion layer includes a plurality of beads.

In some embodiments, the beads include an average diameter smaller thanabout 10 mm.

In some embodiments, the beads include single crystalline silicon,multicrystalline silicon, silicon carbide, or combinations thereof.

In some embodiments, an angle between a pole direction of a first singlecrystalline silicon bead and a normal to the bottom of themulticrystalline silicon ingot is different from an angle between a poledirection of a second single crystalline silicon bead and the normal tothe bottom of the multicrystalline silicon ingot.

Some embodiments of the present disclosure provide a multicrystallinesilicon (mc-Si) wafer. The me-Si wafer includes a percentage of non-Σgrain boundary from about 60 to about 75 and a percentage of Σ3 grainboundary from about 12 to about 25.

In some embodiments, a preferred crystal orientation of the mc-Si waferincludes {112}.

In some embodiments, the me-Si wafer further includes a twin boundary,wherein the percentage of the Σ3 grain boundary is substantially thesame as a percentage of the twin boundary.

Some embodiments of the present disclosure provide a multicrystallinesilicon (mc-Si) wafer. A percentage of non-Σ grain boundary and apercentage of Σ3 grain boundary in the mc-Si wafer are substantiallyidentical.

In some embodiments, the percentage of non-Σ grain boundary and thepercentage of Σ3 grain boundary in the mc-Si wafer are in a range offrom about 40 to about 50.

In some embodiments, a preferred crystal orientation of the mc-Si waferincludes {112}.

In some embodiments, the mc-Si wafer further includes a twin boundary,wherein the percentage of the Σ3 grain boundary is substantially thesame as a percentage of the twin boundary.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A multicrystalline silicon brick, comprising: abottom portion starting from a bottom to a height of 100 mm; a middleportion starting from the height of 100 mm to a height of 200 mm; and atop portion starting from the height of 200 mm to a top; wherein apercentage of incoherent grain boundary in the bottom portion is greaterthan a percentage of incoherent grain boundary in the top portion. 2.The multicrystalline silicon brick of claim 1, further comprising apreferred grain orientation of {112} in the bottom portion, the middleportion, and the top portion.
 3. The multicrystalline silicon brick ofclaim 1, wherein the incoherent grain boundary comprises non-Σ grainboundaries.
 4. The multicrystalline silicon brick of claim 1, apercentage of coherent grain boundary in the bottom portion is lowerthan a percentage of coherent grain boundary in the top portion.
 5. Themulticrystalline silicon brick of claim 4, wherein the coherent grainboundary comprises Σ3 grain boundary.
 6. The multicrystalline siliconbrick of claim 5, the percentage of Σ3 grain boundary being lower thanthe percentage of the non-Σ grain boundary and greater than other grainboundary being more incoherent than the Σ3 grain boundary.
 7. Themulticrystalline silicon brick of claim 3, wherein the percentage ofnon-Σ grain boundaries is from about 65 to about 75 at the bottomportion.
 8. The multicrystalline silicon brick of claim 6, wherein thepercentage of non-Σ grain boundary is greater than a summation of thepercentage of Σ3 grain boundary and the percentage of other grainboundary being more incoherent than the Σ3 grain boundary.
 9. Themulticrystalline silicon brick of claim 5, wherein the percentage of Σ3grain boundaries is from about 12 to about 18 at the bottom portion. 10.The multicrystalline silicon brick of claim 1, further comprising anucleation promotion layer under the bottom portion, wherein thenucleation promotion layer comprises a plurality of beads.
 11. Themulticrystalline silicon brick of claim 10, wherein the beads comprisesan average diameter smaller than about 10 mm.
 12. The multicrystallinesilicon brick of claim 10, wherein the beads comprises singlecrystalline silicon, multicrystalline silicon, silicon carbide, orcombinations thereof.
 13. The multicrystalline silicon brick of claim12, wherein an angle between a pole direction of a first singlecrystalline silicon bead and a normal to the bottom of themulticrystalline silicon ingot is different from an angle between a poledirection of a second single crystalline silicon bead and the normal tothe bottom of the multicrystalline silicon ingot.
 14. A multicrystallinesilicon brick, comprising a plurality of non-Σ grain boundaries; and aplurality of Σ3 grain boundaries, wherein a percentage of non-Σ grainboundaries is from about 60 to about 75 and a percentage of Σ3 grainboundaries is from about 12 to about
 25. 15. The multicrystallinesilicon brick of claim 14, further comprising: a bottom portion startingfrom a bottom to a height of 100 mm; a middle portion starting from theheight of 100 mm to a height of 200 mm; and a top portion starting fromthe height of 200 mm to a top; wherein a percentage of Σ3 grainboundaries is from about 12 to about 18 at the bottom portion.
 16. Themulticrystalline silicon brick of claim 14, further comprising apreferred crystal orientation comprises {112}.
 17. The multicrystallinesilicon brick of claim 14, wherein the percentage of Σ3 grain boundariesis lower than the percentage of the non-Σ grain boundaries and greaterthan other grain boundaries being more incoherent than the Σ3 grainboundaries.
 18. A multicrystalline silicon brick, comprising: a bottomportion starting from a bottom to a height of 100 mm; a top portionstarting from the height of 200 mm to a top; and a nucleation promotionlayer under the bottom portion, wherein the nucleation promotion layercomprises a plurality of beads.
 19. The multicrystalline silicon brickof claim 18, wherein a percentage of incoherent grain boundary in thebottom portion is greater than a percentage of incoherent grain boundaryin the top portion.
 20. The multicrystalline silicon brick of claim 18,wherein a percentage of non-Σ grain boundaries is from about 60 to about75 and a percentage of Σ3 grain boundaries is from about 12 to about 25.